MAJOR: http: change msg->{som,col,sov,eoh} to be relative to buffer origin

These offsets were relative to the buffer itself. Now they're relative to
the buffer's origin (buf->p) which normally corresponds to the start of
current message.

This saves a big dependency between the HTTP message struct and the buffers.
It appeared during this change that ->col is not used anymore (it will have
to be removed). Next step is to turn ->eol and ->sol from absolute to relative.
diff --git a/src/backend.c b/src/backend.c
index f04e26b..f2b564b 100644
--- a/src/backend.c
+++ b/src/backend.c
@@ -254,7 +254,7 @@
 	struct proxy    *px   = s->be;
 	unsigned int     plen = px->url_param_len;
 	unsigned long    len  = msg->body_len;
-	const char      *params = req->data + msg->sov;
+	const char      *params = req->p + msg->sov;
 	const char      *p    = params;
 
 	if (len > req->i - (msg->sov - msg->som))